Various types of actuation systems, such as micro-positioners, translating stages, scanners, or actuated systems with gliding surfaces have been proposed to provide precise positioning of movable components.
One type of device that requires the precise positioning of a movable member is a scanning data storage device that includes one or more heads or tips that move relative to a storage media. Scanning probe storage devices have been proposed to provide small size, high capacity, low cost data storage devices. Actuators are used to effect relative movement between a storage media and an array of tips or heads. The heads and storage media can be located on wafers that are separated by a gap. To achieve a high areal data storage density, the relative displacement of the heads and the storage media should be precisely controlled.
Multiple tips are accessed simultaneously to meet data rate requirements, and these tips form a row of an overall head array. Data are written and read in tracks on the storage media. A row of tips is desirable for a simultaneous reading unit because uniform thermal expansion of the components does not generate off-track motion between tips in a row.
A servo system is used to control the relative position of the heads and the storage media. The servo system can be adversely affected by rotational movement of the media with respect to the head array. Only a small amount of rotation is required to misalign a row of tips, such that the first and last heads in the row are off-track.
In one example, the storage media is mounted on a movable sled supported by springs. Positioning of the media relative to the head array is accomplished using actuators providing linear actuation along the two in-plane axes. Torque can be generated by static misalignment of the geometric centroid and center of mass of the media-sled assembly. In addition, the misalignment can also cause linear dynamic disturbances to have rotational components. Another source of torque disturbance comes from the linear actuation forces or spring attachment points not being aligned with the media-sled assembly center of mass due to manufacturing tolerances. Even a relatively small offset can cause one of the actuators to generate a torque sufficient to cause unacceptable track misregistration (TMR). Other external disturbances include rotational and linear shock and vibration. Furthermore, the presence of head-media friction may result in a steady-state error at the end of a shock event. This steady-state error should also be addressed by the position control system.
While a balancing process during fabrication is likely, it is unlikely that this process will be accurate enough to create a completely balanced system. Given the low rotational stiffiness of existing media-sled assemblies and the extremely tight angular tolerance, relatively minor rotational disturbances can cause an undesirable amount of track misregistration.
It would be desirable to provide active control of the rotation of the movable member to meet system requirements.